The present invention is directed to the design of an implantable defibrillator patch electrode for use with a cardiac pacing and defibrillating device. More particularly, the present invention is directed to the design of an intelligent patch electrode having a number of sensor electrodes. The patch electrode is secured to the exterior surface of the heart to sense electrical activity and deliver an electrical charge to the heart to cause defibrillation.
In order to appreciate the present invention, a fundamental understanding of the physiology of the conduction system in a cardiac cycle is beneficial. Initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. A resulting depolarization wave passes through the left and right atria, stimulating their contraction and producing the P-wave of a surface ECG. This depolarization wave proceeds to the junction of the atria and ventricles. A layer of connective tissue separates the atria from the ventricles and serves as insulation, preventing the disorganized passage of current between atria and ventricles. The atrioventricular (AV) node is the normal electrical conduit between atria and ventricles.
Limiting the current passing through the AV node into the ventricle has two important effects. First, excitation of the ventricle begins at a single point, resulting in an organized contraction pattern. Second, conduction through the AV node is slow, which allows time for the transfer of blood from the atria to the ventricles prior to excitation of the ventricles.
Subsequent depolarization of the ventricles also normally follows an organized sequence. Below the AV node, current passes through the short bundle of HIS, then through the left and right fascicles, and through the Purkinje fibers, leading to depolarization of the large ventricular muscle. The time of conduction through the AV node appears on a surface ECG as the longest part of the isoelectric segment between the P-wave and the QRS complex, with a short conduction time through the HIS-Purkinje system. The delay in conduction of the AV node appears on a surface ECG as the isoelectric segment between the P-wave and QRS complex. The orderly progression of depolarization from AV node through the bundle branches and into the ventricles produces nearly simultaneous contraction of the two ventricles.
In comparison, synchronous contraction results when excitation of the ventricles is abnormal, and the conduction of the depolarization wave is not proceeding according to the above description. A wide variety of illnesses may affect the conduction system, including ischemic, inflammatory and degenerative processes. Idiopathic degeneration of the conduction tissue with fibrosis is a common cause of heart block and sick sinus syndrome.
Additionally, tachycardia is the name given to the condition in which the atria, ventricles or both chambers of the heart beat very rapidly, and not within the normal physiological range, typically exceeding 160 occurrences per minutes. Atrial tachycardia is the medical term assigned to the condition in which rapid and regular succession of P-waves of the PQRST waveform complex occur. The rate of occurrence of the P-waves during atrial tachycardia is in excess of the physiological range normally encountered in the particular patient.
Paroxysmal supra-ventricular tachycardia is the medical term assigned to the condition in which there is a sudden attack of rapid heart condition in the atria or in the atrial-ventricular node. The main characteristics are the same as those in atrial tachycardia.
Normally, atrial tachycardia and paroxysmal supra-ventricular tachycardia are not life-threatening conditions, unless they progress into ventricular tachycardia or fibrillation. Ventricular tachycardia is the medical term assigned to the condition in which rapid and regular succession of R-waves of the PQRST waveform complex occur. Again, the rate of occurrence is in excess of the physiological range of the particular patient and can, if untreated, progress into ventricular fibrillation. In ventricular fibrillation, the ventricles are unable to profuse blood in a coordinated fashion and the heart volumetric output drops to a level dangerous to the patient.
In comparison to the normal cardiac cycle which initiates depolarization at the sinoatrial node, ventricular tachycardia or fibrillation results when a depolarization wave propagation is initiated at one or more additional locations or nodes. Thus, while the sinoatrial node may (or may not) be continuing cyclic depolarization, a second or third node located (for example) in the atrium or a ventricle, will initiate depolarization wave propagations at irregular intervals. It should be understood that once a depolarization wave is initiated, it will propagate in a predictable pattern and at a determinable rate through the cardiac muscle.
Typically, life-threatening ventricular tachycardia or ventricular fibrillation requires immediate treatment by drug therapy or by electrical stimulation, such as cardioversion or defibrillation. Implantable defibrillators were developed to monitor the pacing of the heart, and provide a defibrillation charge via a patch electrode attached to, or implanted in, the heart. Implantable defibrillators require sensing capabilities in order to detect the onset of a ventricular tachycardia or ventricular fibrillation. Thus, a defibrillation system usually includes a transvenously implanted sensing lead which includes sensors positioned within the atrium or ventricle to provide continuous sensory data to the implanted defibrillator. Implantable defibrillators allow the recipient a considerable degree of freedom to pursue normal activities, with the defibrillator monitoring cardiac pacing and providing a defibrillation charge promptly upon confirmed detection of ventricular tachycardia.
Accordingly, for certain patients, it is beneficial to affix to the exterior surface of the cardiac muscle a patch electrode which, when electrically connected to an electrical power source, can deliver a large electrical charge directly to the cardiac muscle to cause defibrillation. The electrical energy necessary for defibrillation when delivered by an implanted patch electrode is in the range of, for example, between 1 and 100 joules, but is preferably in the range of between 5 and 40 joules. It is important to recognize that when this amount of power is being coupled directly to the cardiac muscle, there is a potential for severe damage to the tissue. If such damage occurs, the electrical efficiency of defibrillation from the patch electrode in a subsequent application may be severely impaired.
The design of the patch electrode must allow intimate electrical contact over a substantial surface of the cardiac muscle and provide effective delivery of the defibrillation charge. A further consideration of the design of the patch electrode requires, given its location on the surface of the continuously flexing cardiac muscle, that the patch electrode itself be extremely flexible and resistant to fatigue.
With the foregoing in mind, a patch electrode has traditionally been designed simply as a metallic mesh with a polymer insulation backing and an insulating frame. The patch shapes which have been used include both oval shapes and rectangular shapes. Generally, an oval shape allows more intimate contact with the surface of the heart muscle. The insulating backing is normally bonded to the metallic mesh and operates to direct the defibrillation charge into the cardiac muscle.